The present invention relates to the determination of rotor orientation in brushless DC motors and in particular to the determination of rotor orientation when the rotor is stationary or rotating relatively slowly.
In order to drive a brushless DC motor, it is necessary to detect the orientation of the magnetic axis of the permanently magnetic rotor such that the appropriate driving current may be fed to each stator phase. A stator phase is built up from one or more coils, depending on motor geometry. This can be achieved by the provision of dedicated sensor means, such as a Hall effect sensor. Alternatively, it is possible to determine the rotor orientation without an additional dedicated sensor.
This can be achieved by monitoring the phase inductance which is affected by the rotor orientation.
Monitoring an induced back EMF is not possible when the rotor is stationary e.g. at start up and may also be difficult shortly after start up when the rotor speed is low. In such cases, it is known to determine the rotor orientation by applying pulses to pairs of stator phases simultaneously and determining the sign of the difference in voltage between the stator phases or the sign of the difference in time required to rise to a threshold voltage between stator phases to determine the rotor orientation. Such methods are only ever applied to the resultant inductance along a path consisting of at least a pair of stator phases at any one time. Measuring in this manner using a minimum of two stator phases at every step increases the possibilities for error due to a smaller saturation effect and greater noise. This is a particular problem when the rotor is positioned close to alignment with one of the stator phases because in this case only one phase is being saturated by the rotor flux and the other phases within the conducting path of the applied pulse contribute little to the measured saturation effect. Therefore the measured total inductance variation of the entire path consisting of at least two phases in series is smaller and hence more susceptible to noise.